Power distribution switches are used to couple a power source to an output load device, and monitor and limit the power drawn by the output load device. Typical power distribution switches use current sensing to detect the current drawn by the output load device through the power distribution switch. In high-current applications, the resistance of the power distribution switch is very low, making the insertion of a resistor in the path of the current impractical due to power losses. In such applications, a small field effect transistor and a sense resistor are placed in parallel with the main power FETs of the power distribution switch to sense a small portion of the current. However, the sensitivity of this solution is low. As a result, current limits must be set very high to avoid improper shut down conditions, which poses a potential danger to the power distribution switch.
When adding current limit programmability, the sensitivity of prior solutions becomes even lower. Previous solutions implement current limit programmability by including an adjustable reference current to control the current limit. Because the sense resistor remains constant, the voltage drop over the resistor is reduced as the current limit is lowered programmatically, resulting in an increasing current sensing error due to circuit non-idealities such as transistor mismatches and offsets.